An apparatus for extending the effective exhaust gas path in order to allow gradual diffusion of the exhaust gases in a turboshaft engine wherein the engine includes an annular engine case with at least an exhaust gas duct having an axial component communicating with an annular duct with a substantially radial component and traversing the engine case, the apparatus comprising a collector plenum having a convex annular shape and a diameter larger than that of the engine case with at least one exhaust outlet defined in the plenum and wherein the annular duct extends into the plenum beyond the diameter of the engine case to extend the exhaust gas path.